Potassium nitrate can easily be synthesized by reacting potassium carbonate with nitric acid:

K2CO3 + 2HNO3 -> 2KNO3 + H2O + CO2

In this case, the synthesis also served to approximately determine the content of potassium carbonate in the solid product obtained from wood ash by extraction.

For the synthesis, along with the potassium carbonate and nitric acid, it was necessary to prepare a few glass jars, and a wider tray or a container in which the solvent from the final solution can rapidly evaporate.

First, 381 grams of the impure potassium carbonate were taken and dissolved in a minimal amount of water. The dissolution was very exothermic, so a lot of heat was generated during this process.

After that, the nitric acid (w=67%) was slowly added. The reaction was very vigorous, so the acid was added drop by drop, through a longer period of time.

When there was no visible reaction upon further addition of the acid, there was no more K2CO3 left in the jar, which was also confirmed by checking the pH. The obtained solution was then poured into a tray and left for some time, so that the water could evaporate.

On the image above, one can see the obtained potassium nitrate crystals after the first recrystallization. The mass of the product was equal to 485.3 grams (4.8 moles). According to the chemical reaction equation, the real amount of potassium carbonate had to be half of the amount of potassium nitrate, i.e. 2.4 moles. When one calculates the mass from the amount of K2CO3, one can see that 381 grams of the impure reagent, contained 331.7 grams of pure potassium carbonate. Thereby, there was approximately 87% of potassium carbonate in the product obtained from wood ash.